Analogue to Digital Converters (ADCs), have been found wanting in the RF area where they are used to convert analogue narrow band IF in a receiver to the digital domain so that demodulation can be performed in ways that can not be achieved using traditional analogue equivalents.
The following description uses the RF area as a good example of an application of the beneficial application of ADCs but it will be apparent to those skilled in the art that the real advance disclosed herein is the novel approach of using an ADC array and the provision of a method to nullify the effects of the inherent non-linearity of ADCs.
Analogue RF front ends for receivers are still preferably used today since:                1. the dynamic range of wide band digitising systems are seen as inferior to that obtained from analogue filters, and        2. the signal processing needed for wide-bandwidth processing is deemed to be both beyond the capacity of affordable DSP engines and unnecessary for an individual receiver which normally attempts to tune only one signal at any instant in time.        
During the recent past, these beliefs have become less true, but the highest performing receivers are still undoubtedy analogue superheterodynes that subsequently use digital demodulation techniques (i.e. an analogue front-end with the more flexible digital backend).
Since most users look at the performance of individual receivers, the conventional wisdom is that direct digitising receivers are inferior.
However, a receiver site that shares multiple antennae amongst multiple receivers can make the direct digitizing approach competitive with and potentially exceed conventional analogue systems.
In a large analogue receiver system, there are a number of antennae shared by a number of users. This results in a need to split the incoming RF energy from all the antennae so that any one operator can receive a desired frequency using one or more beams pointing in roughly the right direction. However, every time the received signal is split there is a loss in fidelity (i.e. the Signal to Noise Ratio (SNR) gets a little worse). Further, when independent signals are combined to give directional beams, the finite accuracy of the combiners (typically 2 degrees at 0.1 dB) means that the beams are not precise. Moreover, this approach requires a dedicated receiver for every user, thus users must accept the “best available” beam even if it's not exactly appropriate at the time and even if an unwanted signal (interference) is also on that channel at that time.
Despite these compromises, analogue techniques work. It is however unlikely that any major advance in performance using these techniques is achievable as this technology is well developed.
In the analogue domain, the overall system performance is limited by the performance of the RF front end. However, it is proposed that the use of a suitable analogue to digital architecture can create a digital domain in which appropriate use of signal processing techniques can compensate for many of the imperfections of the conversion process in the front-end thus overall enhancing the system performance.
A single ADC handling two large in-band signals F1 and F2 will, due to non-linearity in the ADC, particularly in the Sample and Hold, produce spurious harmonics and intermodulation products in the output spectrum at F1+F2, 2F1−F2 etc. Thus any small signal on those particular frequencies would be difficult if not impossible to detect or copy, where copy is used to mean reception and demodulation of the transmitted signal without error. Where these harmonics and intermodulation products lie above the Nyquist frequency, they will appear as aliased in-band components with the output of the ADCs. Thus, the largest intermodulation products are considered to limit the useful dynamic range of an ADC.
In an example, a more realistic situation in the H.F. band would be 50 large signals each being 3 kHz wide. This would generate about 2000 in band 2nd order, 5000 in band 3rd order intermodulation terms. Making some reasonable assumptions of the degree of overlap from the intermodulation products and also realising that the 2nd order products are 6 kHz wide, 3rd order 9 kHz etc., the total bandwidth corrupted by spurious signals occupies about 25 MHZ of the nominal 30 MHz bandwidth for H.F. Thus, the intermodulation performance determines the performance of a single channel-digitising receiver.
However, if an array of these ADCs were connected to a spatially dispersed antenna array, their largest intermodulation products no longer constitute a performance limit. We can assume that the same two large signals we will still experience the same spurious frequencies from each ADC, but each would have their own apparent angle of arrival. Hence, the Signals Of Interest (SOI) from other directions can still be readily separated using conventional co-channel separation techniques, even if the SOIs are much smaller than the interfering spurious signal.
Using the above approach, it is possible to accept the limited spurious free dynamic range (SFDR) of individual ADC channels induced by ADC non-linearities, but provide a higher system SFDR by using advanced signal processing techniques to remove the spurious signals and so allow small signals which are below the noise floor of individual ADCs to be received for analysis and copy. This approach effectively ‘linearises’ the non-linear ADCs.
Thus the use of an array of ADCs separated either spatially or with filters having a variable phase delay can provide a higher dynamic range Analogue to Digital conversion than previously achievable thereby offering an alternative arrangement for the use of ADCs than was previously available and also allows for the use of lesser quality ADCs in an arrangement that provides equal or better performance than a single ADC of higher quality.